The 3rd Generation Partnership Project (3GPP) started a Release 13 Long Term Evolution Advanced (LTE-Advanced) study item, Licensed Assisted Access (LAA), aiming to use the unlicensed spectrum based on LTE technologies, on which WiFi is currently deployed. LAA carrier is always carrier aggregated with a licensed carrier where LAA carrier is a secondary carrier and the licensed carrier is a primary carrier. It is observed that LTE significantly impacts WiFi performance in LTE-WiFi coexistence case, if current LTE functionalities are assumed. One major reason is that WiFi follows Listen-Before-Talk (LBT) principle, which specifies that a WiFi Node can only start transmitting after it has performed Clear Channel Assessment (CCA) and measured that the channel is idle, while a legacy LTE Node does not perform CCA and may transmit continuously. The continuous transmission from LTE may make the WiFi nodes always measure there are other Nodes transmitting, i.e. collision happens, and the WiFi nodes tend to transmit with significantly less possibilities, resulting in degraded performance. To ensure fair co-existence with WiFi, LTE needs to be modified to also support LBT on the unlicensed band.
To ensure fair co-existence with WiFi, it is agreed for LAA to support LBT and discontinuous transmission as well as limited maximum transmission duration on a carrier in the unlicensed band. The LAA eNodeB can only start transmission when the channel is clear as measured by Clear Channel Assessment (CCA). After a transmission of limited maximum duration, the LAA eNodeB needs to release the channel and perform CCA again to use the channel, resulting in opportunistic transmission with maximum transmission time of around 13 ms for Load Based Equipment (LBE) and 10 ms for Frame Based Equipment (FBE). The gap length between two transmissions could be variable and is affected by other transmitting Nodes, e.g. WiFi Nodes and LAA eNodeBs.
For LBE, CCA is minimum 20 μs, extended CCA (eCCA) duration is a random factor N multiplied by the CCA time, where N is randomly selected in the range 1 . . . q every time, q=4 . . . 32, and Channel Occupancy Time is <=(13/32)×q ms. For FBE, CCA is minimum 20 μs and performed in the end of IDLE period, Channel Occupancy Time is 1 ms at minimum and 10 ms at maximum, IDLE period is Minimum 5% of channel occupancy time and Fixed Frame Period=Channel Occupancy Time+IDLE Period.
A fundamental procedure in a cellular wireless system is synchronization, which is performed by a mobile terminal for obtaining time- and frequency synchronization to a cell in the network and detecting its cell identity. LTE synchronization signals are often transmitted periodically by the base station, e.g., in subframe 0 and subframe 5 for FDD. However, given the opportunistic nature of transmission on the unlicensed band, LAA Nodes following Listen Before Talk (LBT) principle are not able to transmit on the unlicensed band continuously. When an LAA node is allowed to transmit after it succeeds in measuring the channel is idle, the time gap between this transmission and the last transmission could be long, and during the gap the receiving Node is not able to achieve or maintain the time- and frequency synchronization since nothing is transmitted. The synchronization between the transmission Node and the receiving Node could be already lost due to mobility and frequency drift. Typically, a control channel is transmitted before the data channel and contains necessary information for receiving the data. Synchronization will have to be established before the receiving Node is able to demodulate any control channel. Depending on when the transmitting Node has found the channel to be clear, the transmission of a synchronization signal may occur at an instant which is not known to the receiving Node. Therefore, the receiving Node may continuously need to try detecting the synchronization signal.
In LTE, synchronization signals, i.e. Primary Synchronization Signal (PSS) and Secondary Synchronization Signal (SSS), are sent periodically in every radio frame. The UE performs initial synchronization (i.e., identifies a cell) based on PSS/SSS and performs Radio Resource Management (RRM) measurements on the Common Reference Signal (CRS). It may also perform synchronization tracking based on downlink signals, e.g. PSS/SSS, CRS or Channel State Information-RS (CSI-RS). For LAA secondary carriers, initial synchronization may be performed based on the periodic signals as well. One possibility is opportunistic periodic PSS/SSS transmission. That is, the PSS/SSS are transmitted in a set of predefined subframes (e.g., subframe 0 and subframe 5 for FDD) but are only transmitted if the LAA eNodeB measures the channel as clear. In addition, some regions (e.g. using ETSI standards) allow short control signalling transmitted with maximum duty cycle of 5% every 50 ms without the need of CCA. One other possibility is therefore that PSS/SSS are sent with a larger periodicity than 5 ms, e.g. as part of short control signalling. The User Equipment (UE) may perform initial synchronization based on this kind of less frequent synchronization signals. It shall be kept in mind that the synchronization design shall not always assume the existence of short control signalling because it is not globally allowed.
After initial synchronization is achieved, the UE may lose the synchronization to the LAA eNodeB after a transmission gap of uncertain length between the previous transmission and the current transmission. The loss of synchronization may be caused by the oscillator drift and the mobility. Note that the UE may not be able to rely on the primary carrier frequency tracking because the Doppler frequency shift could be quite different on the primary carrier and the secondary carrier due to significantly different carrier frequencies. In addition, the primary carrier downlink timing would be different with the secondary downlink timing in some deployment scenarios, e.g. non-collocated macro LTE eNodeB and LAA eNodeB case. Thus each carrier should transmit the necessary synchronization signals for fine synchronization or synchronization tracking.
Relying only on the periodic synchronization signals for LAA fine synchronization or synchronization tracking could be very inefficient in terms of resource utilization. For example, if the LAA eNodeB measures the channel as clear at a time which is not defined for PSS/SSS transmission, say subframe 1 for FDD, the LAA eNodeB may have to wait for up to 4 ms for PSS/SSS before scheduling data transmission, which is a severe waste of time resource especially considering the limited maximum transmission time on LAA carriers.
Meanwhile, relying on the potential new periodic signals, i.e. synchronization signals sent as part of short control signalling of periodicity of tens of ms, would be even more inefficient as the LAA eNodeB may have to wait for an unreasonably long gap for synchronization signal transmission.
With conventional solutions the LAA receiving Node may not be able to achieve synchronization when the LAA transmitting Node wins the contention/CCA and is ready to transmit, because the LAA transmitting Node may not be able to transmit synchronization signals periodically as in LTE Rel-8. Thus the LAA receiving Node may be prohibited for obtaining synchronization and therefore not be able to receive data instantly.
With conventional solutions there would be a waste of OFDM symbols if there is a time gap between when the transmitting Node wins the contention/CCA and the specified OFDM symbol to carry PSS and SSS. Thus, the spectral efficiency of the system will deteriorate.